1. Field of the Invention This invention relates to the identification of on-hook and off-hook states of a telephone subscriber loop, and in particular to apparatus for detecting such states and providing the status information to a local switching office.
2. Background Description
The problem of supplying operating power from a central office to a subscriber telephone set is a continuing one. The arrangements for doing so are variously described as line feed circuits and telephone subscriber line circuits. For the standard telephone circuit te technique for supplying the operating current to the subscriber loop consisted essentially of two resistors which are designated as line feed resistors and these resistors are connected to opposite sides of the central office battery. The voltage amplitude of the battery is in the range of 48 to 60 volts and the line feed resistors typically have resistance values ranging from 200 to 800 ohms depending upon the particular telephone exchange. These line feed resistors are selected at the central office to have a value which would produce the proper line current for the loop resistance characteristic expected by the telephone instrument to which it is to be connected.
Another common technique is by the use of a transformer, the primary of which is connected to the voice frequency circuit. The secondary consists of two windings, the two secondary windings being joined by a blocking capacitor, which provides a low AC impedance for voice frequencies and a high DC impedance. The battery voltage is applied to the one and other terminal ends of the capacitor. One problem with this approach is that loop supervision, i.e., on-hook, off-hook and dial pulsing information could not be detected by using the battery feed circuit. Thus, separate detection circuits are required. These traditional circuits are not energy efficient, because of the excessive loss in the line feed resistors.
More recently, floating battery feed circuits, which are more energy efficient, have been devised. One such circuit is disclosed in the U.S. Pat. No. 4,056,691 entitled, "Telephone Subscriber Line Circuit", L. Freimanis et. al., granted Nov. 1, 1977. In the referenced patent it is clear that the battery feed current is coupled to the telephone instrument from a source of positive and negative potential that "floats" (has a very high impedance) relative to earth ground. The main advantage of this method is that of very good longitudinal balance, which provides good immunity to AC induction. As shown in FIG. 3 of the referenced patent a transformer is included which has, in addition to the normal primary and secondary windings, a plurality of sense windings which are used to ensure a constant current supply to the subscriber loop when the battery feed circuit is turned on. Further, a number of external control circuits are required. For example, a scan flip-flop is used for status checks.
In order to determine the status of a subscriber line, the scan flip-flop is placed in the set state by a controller which is a part of the telephone central office. Off-hook scanning is done at a low frequency to reduce power consumption so the circuit (power circuit) will be activated on a periodic basis by a clock signal. Thus power is supplied to the line at a low periodic rate and a separate sense winding is used to respond to the line voltage and this voltage is applied to a comparator. In the off-hook state the sensed voltage is relatively low, because of the lower impedance presented by the closed loop. The voltage will be considerably higher in the on-hook state when the line impedance is high. Because the sensed voltage could become excessively high under this latter condition, the sense winding is clamped to a supply voltage through a light emitting diode.
In the instant invention the on-hook and off-hook state are identified without requiring a separate scan flip flop, sense winding or clamping circuit.